When you see an extremely high total in the NFL (Over 54), and a single digit line, you can be assured that the dog has a very good chance of covering for you (10-0 ats since the 2001/2002 season). Actually, the lower the line, the bigger the chance for the dog to win the game. (5-0 when not getting 4+ pts).
When you see an extremely low total in the NFL (Under 33), the favorites have a very good chance of covering as lons as the line is not extremely high or extremely low (the line has to be in between -3 and -13). Their record in this situation is 36-19 ATS.
Both angles apply to regular season only as obviously the totals are much lower in the pre-season.
Extremely big favs (these are not very oftenly seen, last season the Pats were the only ones and they went 0-4 ATS in this situation) of at least 17 pts are 4-20 ATS.